Since the deployment of the United States Navstar Global Positioning System (GPS) in 1984 and the 2000 Presidential Order disabling Selective Availability for civilian uses, precise location has been available to those equipped with GPS receivers.
The location services marketplace includes individual consumers who want to track their family members, pets, or physical assets; business owners who want to track their employees or physical assets; caretakers of Alzheimer's patients; owners of vehicles at risk of theft; and the like. Any mobile entity capable of carrying a locator device, incorporating the locator device circuitry or having the capability to attach the locator device can be situated inside or outside a geo-fence and the user of the locator device may be notified upon the device's exit or entry of a pre-provisioned 2-dimensional geographic zone area or geo-fence.
A geo-fence is a horizontal boundary on the ground based upon a closed polygon, for example a circular radius around a fixed geographic point or a multi-point polygon encompassing an area-of-interest and/or excluding areas such as roadways or properties. For the simple circle boundary, the Haversine formula is used in calculating the “great circle” distance from the locator to the boundary. The Haversine formula is an equation for calculating the great circle distances between two points on a sphere using their longitudes and latitudes. For the polygon boundary, defined as a set of up to n points, there are an equal number of arc segments. After the segment point closest to the locator is selected, the Haversine formula is applied.
A locator geo-fence consists of a boundary and an attribute to indicate when a notification will be sent—entry, exit, or both. One approach is to make use of the emerging Assisted GPS (AGPS) technology to obtain a highly reliable fast location determination that includes a trustworthy error ellipse (for convenience, of equal major and minor axes). Treating the location as a circular field of radius r rather than just a point, geo-fence factors have been applied to arrive at an acceptable balance of certainty and sensitivity for notifying subscribers that their locator has entered or exited a zone.
Practical experience in the limitations of the GPS solution reveals that an approach conforming to actual GPS behavior works much better than relying on the manufacturer specification sheets. For example, when a GPS chipset supplier specifies its accuracy, it is typically specifying open sky conditions and 95% confidence, meaning that with no obstructions 5% of locations will fall outside that error radius, and no promise of any absolute maximum error. Adding obstructions drops the confidence to 2-sigma (86.5%) or less. Without any means of minimizing false positives, false notifications may occur 13.5% or more of the time.